MEMS devices are becoming increasingly popular. MEMS transducers, and especially MEMS capacitive microphones, are increasingly being used in portable electronic devices such as mobile telephone and portable computing devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more moveable membranes and a static backplate, with a respective electrode deposited on the membrane(s) and backplate, wherein one electrode is used for read-out/drive and the other is used for biasing. A substrate supports at least the membrane(s) and typically the backplate also. In the case of MEMS pressure sensors and microphones the read out is usually accomplished by measuring the capacitance between the membrane and backplate electrodes. In the case of transducers, the device is driven, i.e. biased, by a potential difference provided across the membrane and backplate electrodes.
To be suitable for use in portable electronic devices such transducers should be able to survive the expected handling and use of the portable device, which may include the device being subjected to loud noises and being accidentally dropped. These events may result in a high pressure impulse incident on the transducer. It has been found that in known MEMS transducers high pressure impulses can potentially lead to damage of the transducer.
To help prevent any damage which may be caused by these high pressure impulses it has been proposed that the MEMS transducer could be provided with at least one variable vent which can provide a variable size flow path between the front and back volumes. In a high pressure situation the variable vent(s) provide a relatively large flow path between the volumes so as to provide for relatively rapid equalisation between the volumes, reducing the extent and/or duration of a high pressure event on the membrane. At lower pressures however, within the expected normal operating range of the transducer, the size of the flow path is minimal or negligible.
The variable vent structure thus acts as a type of pressure relief valve to reduce the pressure differential acting on the membrane at relatively high pressure differentials. Such variable vent structures can thus be very useful for providing MEMS transducers, especially microphones, that can better survive high pressure events.
The frequency response of a microphone characterises its sensitivity level across the frequency spectrum. MEMS microphone performance can be measured by the frequency response i.e. the change in sensitivity at various frequencies. A flatter, linear, characteristic across the frequency range of interest is considered to be indicative of superior microphone performance. A typical frequency response figure shows an actual microphone's response across the frequency band.
FIG. 1 illustrates an example of a MEMS microphone frequency response. From FIG. 1 it can be seen that the MEMS microphone exhibits a generally flat frequency response characteristic across a wide band of frequencies but that the frequency response exhibits a drop, or “roll-off”, at low-frequencies below 75-100 Hz. In other words, the sensitivity of the microphone becomes non-linear at low-frequencies. The frequency response characteristic also exhibits a resonant response at high frequencies above 10 k Hz.
The high/low frequency limits are described as the points at which the microphone response is 3 dB below/above the reference output level at 1 kHz. The reference level at 1 kHz is customarily normalized to 0 dB. The frequency response characteristic also includes the deviation limits from a flat response within the pass band. These values, expressed as units of ±x dB, show the maximum deviation of the output signal from a typical level between the −3 dB points. The −3 dB roll off frequency fc is defined as:fc=1/(2*PI*Air flow resistance through the diaphragm*back volume compliance).
The frequency response of a MEMS microphone varies due to a number of parameters including the location and/or geometry of the front and back chambers, and of the ventilation hole. Frequency response is also a function of the mechanical properties and compliance of the diaphragm.
It may be desirable to adjust or tune the frequency response of a MEMS transducer, in particular to tune the low-frequency response according to the intended or desired characteristics of the device.
Any venting or leaking through a membrane will cause a shift in the low-frequency roll-off point. In view of this, it is known to provide one or more openings or holes in the membrane layer of a MEMS transducer for tuning, or adjusting, the low-frequency response of the transducer. In particular, the −3 dB point may be adjusted by controlling the size and number of openings and thus the amount of air flow through the membrane.
A number of problems are associated with providing one or more openings in the membrane, according to previously considered transducer designs. In particular, any perforation in the membrane, particularly in the central region of the membrane where the membrane will undergo the greatest deflection in response to an incident differential pressure, may tend to weaken the membrane structure. Thus, the provision of openings for low-frequency tuning in the central region of the membrane may potentially undermine the robustness of the MEMS transducer, particularly in the case of MEMS transducers which require a significant number of openings. The openings may be located laterally outside the central region of the membrane towards the periphery of the membrane, however it will be appreciated that space is limited in this region. Furthermore, since the openings may alter the localised stress gradient exhibited by the membrane, leading to non-uniformities in the stress distribution, openings in the periphery of the membrane may potentially give rise to edge curl, which may degrade the performance of the microphone.
Embodiments of the present disclosure are directed to methods and apparatus for mitigating the problems associated with providing one or more openings in the membrane for tuning the low-frequency response of the membrane.